Presently, one of the difficulties faced by manufacturers is the time lag between concept and development of a new technology and the introduction of actual products to the market. In manufacturing, a critical time-limiting step for many products is the design and fabrication of molds and dies. Complex dies may take from weeks to a year to perfect prior to manufacture of a product. In present manufacturing processes, added steps are necessary to overcome deficiencies of present fabrication methods. For example, for molds and dies, products must be machined to provide cooling channels and acceptable surface finish.
Known processes which deposit metal result in a sintered product, due to trapping of oxides and inadequately bonded material. Even in the case where acceptable material deposition has occurred, the process often entails the buildup of stresses which must be relieved. One such known process is laser cladding, wherein a laser is used to generate a melt-pool on a substrate material while a second material, typically a powder or wire, is introduced, melted, and metallurgically joined.
Cladding is generally distinguished from alloying on the basis that cladding melts a relatively small amount of the base substrate material relative to the amount of the deposited material, and the powder system delivers a controlled volume of metal particles into this molten volume. The particles become dispersed throughout this molten volume and form a deposition of a desired composition on the outer layer of the substrate. Removal of the laser beam from the molten volume, such as by advancement of the substrate workpiece relative to the focal point of the beam, causes the molten volume to be rapidly chilled. The chilling occurs so rapidly that the volume often retains the characteristics of the molten mix.
Conventional laser cladding techniques move the metal article relative to the focal point through the use of jigs, parts handlers, and the like. The beam focal point therefore remains fixed in space, as does the powdering point. Uniform movement of the metal article usually requires a complicated jig which is difficult to manufacture, very expensive, and usually not very successful, particularly with intricate geometries. For this reason, laser cladding of metal parts having other than relatively flat geometries have been nearly impossible to achieve on a consistent uniform basis. To the present time, it has not been possible to control the dimension and properties of the deposit. Close control of dimension is necessary in order to apply the basic cladding technique to the production of parts having close tolerances, acceptable microstructures and properties, and which can be produced at a reasonable cost and within a reasonable period of time.
The present invention is useful in automatically controlling the build-up of material on a substrate, and is particularly useful in fabricating metal parts through repetitive cladding operations as might be required for small volume manufacturing, prototype runs, and the like. Broadly, and in general terms, a laser is used to locally heat a spot on a substrate, forming a melt pool into which powder is fed to create a deposit having a physical dimension. Optical detection means coupled to an optoelectric sensor are used to monitor a physical dimension of the deposit, and a feedback controller is operative to adjust the laser in accordance with the electrical signal, thereby controlling the rate of material deposition.
In the preferred embodiment, the physical dimension is the height of the deposit, and the system further includes an interface to a computer-aided design (CAD) system including a description of an article to be fabricated, enabling the feedback controller to compare the physical dimension of the deposit to the description and adjust the energy of the laser in accordance therewith.
In terms of specific apparatus, the optical detection means preferably includes an apertured mask through which light from the deposit passes to reach the optoelectric sensor, and the feedback controller includes circuitry for adjusting the laser in accordance with the presence or absence of the light from the deposit.
A system for automatically fabricating an article according to unique features of the invention would comprise a computer-aided design database including a description of the article to be fabricated, a work table for supporting the substrate, and translation means for moving the substrate relative to the laser and feeding means. In one arrangement, the translation means moves the work table while the laser and feed means remain stationary, whereas, in a different configuration, the translation means moves the laser and feed means while the work table remains stationary. As a further alternative, both the laser/material feed and work table/substrate could be moved simultaneously, preferably under feedback control.
A process of fabricating an article according to a method aspect of the invention would include the following steps:
providing a description of the article to be fabricated; PA1 providing a substrate upon which to form the article; PA1 heating a localized region of the substrate to form a melt pool thereon; PA1 feeding material into the melt pool so as to create a deposit having a physical dimension; PA1 optically monitoring the physical dimension of the deposit; PA1 controlling the physical dimension in accordance with the description of the article to be fabricated; and PA1 advancing to different localized region of the substrate until the fabrication of the article is completed.